1. Field of the Invention
The present invention relates to a dual switching power supply (SPS) system and more particularly to the control circuit of a dual switching power supply system.
2. Description of Related Art
A dual switching power supply system is employed when an electronic system requires a variable voltage as an input. For instance, a monitor system which may accept multiple frequencies requires different horizontal deflection voltages to correspond to the different horizontal synchronization frequencies of the input video signal. In situations such as this, a dual switching power supply system is generally used. A first set of switching power supplies is used to generate fixed voltages required to power the different elements in the monitor system. A second set of switching power supplies is used to generate a variable voltage needed.
In accordance with most conventional systems, the control circuit of the first SPS set is independent from that of the second SPS set. The second SPS set is activated and outputs power only after the first SPS set is activated and outputs power normally.
In order to provide reference voltages, to provide an oscillation signal, to drive the switch such as a MOS transistor, to provide amplifier function and compensation etc., a commercially available integrated circuit (IC), such as Part No. UC3842 from S.T. Co. or Unitrode Co., is most frequently used in the design of the control circuit of a switching power supply system. In this technical field, the UC3842 chip is referred as a Current Mode Pulse Width Modulation Controller. This kind of control IC is designed such that it is only activated when the power supply is increased to a voltage called start-up voltage. The start-up voltage of the UC3842 is about 17.5 volts. However, after the UC3842 is activated, it may operate as expected even if the power supply drops below start-up voltage, as long as the power supply is above a shut-down voltage. The shut-down voltage of the UC3842 is about 8.5 volts. Further information on UC3842, is publicly available.
In a conventional dual switching power supply system, the auxiliary power output of the first SPS set is provided not only to power the control IC itself, but to power the second set of the control IC. As a result, the auxiliary power output must be equal to or greater than the start-up voltage. In the system of this kind, not only is the voltage stress the electronic element takes higher than desired but so is the power loss, due to the higher operation voltage which is equal to or greater than the start-up voltage. Also, due to the higher operating voltage, critical components such as a power MOS are more easily broken down if the switching power supply operates abnormally, such as with an over current or dead short circuit of the output terminals of the transformer. Such critical conditions can only be avoided through implementation of a protection circuit which requires additional efforts and costs.
As shown in FIG. 1, the conventional dual switching power supply system comprises a first transformer set 111 and second transformer set 112. The first transformer set 111 has a primary coil 113 which is selectively energized through control of the ON/OFF state of a first switch 115. The second transformer set 112 has a primary coil 114 which is selectively energized through control of the ON/OFF state of a second switch 116. The first and second switch have first and second control input terminals respectively.
The control circuit of the dual switching power supply system, as shown in FIG. 1, in general, comprises a first control circuit 11, second control circuit 12 and associated circuits. The first and second control circuits 11, 12 may be off-the-shelf integrated circuits available in the marketplace. For instance, a Part No. UC3842 control IC manufactured by Unitrode Co. is frequently used in the design of the control circuit of a dual switching power supply system. The functions of the control IC, such as UC3842, at least include providing a reference voltage at the output pin 8, providing an oscillation signal to operate the internal circuit of the control IC and therefore to generate the switching signal needed at the output pin 6 and providing amplifier function and feed forward compensation. The pin 7 is an input Vcc pin which is coupled to an input direct current source B+. As the control IC's 11, 12 are implemented in the control circuit of the switching power supply system, the output signals of the output pin 6 are coupled to the first and second control input terminals of the first switch 113 and second switch 116 respectively. According to the conventional design, the output signal Va of a primary output coil 117 of the first transformer set is coupled to the node C of the control circuit 11 through a diode 120, and the output signal Va is input to the input Vcc pin 7 of the control circuit 12. The power B+ functions to start up the control IC 11. The control IC such as UC3842 is designed in such a way that it is activated when the input voltage to the Vcc pin 7 is increased to a voltage called start-up voltage. After the UC3842 has been activated, it may operate as designed even if the input voltage at the node C drops below the start-up voltage, as long as the input to pin 7 is still above a shut-down voltage. The function of the capacitor 118 is to store a charge during the time when the power B+ is sourcing the current into the control circuit 11. The capacitor 119 is used to store the charges during the time when the output of the primary output coil 117 is sourcing current into the control circuit 11. The diodes 120 and 121 are used to prohibit the charges of capacitors 118, 119 from being discharged in the right hand direction. The components of the control circuit, such as the resistor R1, etc. in FIG. 1 are chosen in a way such that the response curve of the voltage at the node C has the shape shown in FIG. 7. The Vst is the start-up voltage required to activate the control IC 11 or 12. The Vsh is the shut-down voltage than which the input voltage to the pin 7 of the control IC must be greater for the control IC to function. As shown in FIG. 7, during the period of t0 to t1, the input B+ provides currents to the control IC 11 and the capacitor 118 builds up its voltage. During the period of t1 to t2, the control IC sinks additional currents from the capacitor 118 and the voltage at node C is decreasing. During this time, the diode 120 is not conducting. At a time just before t2, the control IC 11 starts to output the switching signal to the gate of the switch 115. Therefore, the primary output coil 117 begins to provide voltage and current to the associated components and the diode 120 is conducting. As a result, after t2 the voltage at the node C begins to increase to a steady state voltage value due to the charging phenomenon of capacitors 118, 119 by the output current of the primary output coil 117. The steady state operating voltage of the conventional design in FIG. 1 must be 17.5 volts at least. After the first transformer set 111 and associated circuits function normally, the control IC 12 commences to activate due to the input voltage Va at the pin 7 which is substantially 17.5 volts. It is obvious that, in the conventional design like that in FIG. 1, the voltage stress the components take is high and the power loss, as a result, is also substantial. In addition, once there is an abnormal operating situation, such as an over current a or dead short of the output terminals of the transformer set, it is far more safe to all components in control circuit if the operating voltage drops immediately below the shut-down voltage which is 8.5 volts for UC3842. However, the voltage at the node A or C can hardly drop to a level below the shut-down voltage according to the conventional design as shown in FIG. 1.